1. FIELD OF THE INVENTION
The present invention relates to load detection devices, and more particularly to an improved load sensing device adapted for attachment to a load carrying line of a lifting crane for accurately sensing the tension in the line.
2. DESCRIPTION OF THE PRIOR ART
To operate and protect lifting cranes, such as those used in the material and cargo handling and construction fields, it is desirable to continually measure the tension or loading that exists in the crane's lifting ropes or load lines, which are generally steel strand wire ropes designed to support large weights. Not only is it important to provide for warnings of an overload condition, but it is also useful to provide indications of loading changes, and to detect conditions where the loading is released from the wire ropes, such as when a cargo is set down at a location out of view of a lifting crane operator.
Load line tension is generally measured using one of two types of measuring devices: An in-line type of sensing device which interrupts the rope or attaches at a rope dead end connection so as to act directly in the rope's line of force, or a sensor of a rope deforming type which operates on the principle that the restoring force in a load line deformed from its straight line condition by an external load line operator will provide a detectable force on the operator proportional to the tension in the line. In-line devices can be relatively expensive or impractical to install because of the need to break the rope for its installation. However, devices of the rope deforming type have the disadvantage of being relatively inaccurate when compared to in-line devices.
The problems of line deforming sensors are largely due to friction caused by relative movement of the load line and the line contacting sensing device, but can also result from other extraneous effects such as temperature variations and structural arrangement. As a specific example, a device has previously been designed for a static wire rope wherein the wire rope is secured by the two extreme ends of the device, with one end being a sliding securement; the rope is deformed in the middle between the two securements by a load sensing unit which cradles the rope on a suitable curved support surface. It is believed that the relative inaccuracy of this device is primarily caused by the relative movement of contact surfaces which occurs with a change in tension in the rope.
The present invention is an improvement on existing load sensing devices of the line deforming type, and particularly, those installed on a static wire ropes. The invention substantially overcomes the friction problems and other extraneous influences affecting the accuracy, sensitivity, and repeatability of conventional sensing devices by providing a load sensing device which is compact and eliminates sliding contact between the device's structural elements and the wire rope. The invention also has the advantage of being easily installed and removed from any wire rope that can be slackened to a condition to permit clamping and is adapted for use on all sizes and types of ropes and to most any loading capacity and condition.